Laos–Thailand border

The river line

In a treaty with France in 1893, Siam gave up all its rights on the left bank of the Mekong and to every island in the river, and accepted a 25-kilometre demilitarised zone on the right bank. In 1904 France also took the right-bank territories of Luang Prabang in the north and Bassac in the south, where the line follows mountain water divides; a treaty of 1907 confirmed the arrangement.^[1]

A convention of 1926 set the line in the Mekong: it follows the main channel, but where there are islands it takes the channel nearest the Siamese shore, so the islands go to Laos. Because the Mekong rises and falls so much, many islands become attached, or nearly so, to the Thai bank at low water, and the river boundary then turns into a land boundary. About 600 miles of the border lie in the Mekong, and about 404 miles follow water divides.^[1]

“The boundary, as a result, could and does become a “land boundary” during certain times of low water.”^[1]

Friendship bridges

Until 1994, goods and people moving between Vientiane and Nong Khai had to go by ferry. The First Lao–Thai Friendship Bridge, funded and built by Australia, opened on 23 April 1994 as the first major land link between the two countries.^[2] Others followed at Savannakhet–Mukdahan in 2006, Khammouane–Nakhon Phanom in 2011 and Houayxay–Chiang Khong in 2013; a fifth, between Bolikhamxay and Bueng Kan, opened in December 2025, and a sixth is planned.^[3]